Most people liic in homes that are firmly fixed in one spot. They may be huge apartment blocks or small private resicences, mud-walled huts or houses on stilts, but they are alike mn that they arenot easily moved. For some people however, who like to wander, a permanent dwell ng isa handicap. They prefer a mobile home, one that they can pick up and take along with them on their travels. Tents are con eniently portable, and for many ocnturies they have been a favourite form of shelter tar nomadic tribesmen. Tents are also froquently used in military operations, and on ordinary camping trips other types of mobile homes, such as caravars or houseboats, are nxurner and more omfortable to ive in than canvas tens. certain anuntals also enjoy the advantages c Living in a mobile baume. The snail, which carries its tikuuse about en is beck. manufactures for itself a durable shelter cf exactly the nght size. As the snail grows, il adds Loits shell, that never needs to sear about for larger quarters. A snail does nok shed orexchange its shell: it simply goes on enlarging it. The shell is composed of a lime liko substance secreto by a fold of skin called the e nkantle cumpletely lines he shell and wraps around the snail'sbody. Sare ans flow fin the interior ma gin of the mantle throughout the lifo of the smail, and then evenness of 1he flow es the smoutiness of the shell. An even flow produces a smooth, thin hell while an uneven flow results in ridael and Lairly thick Rhell. The smail begin oonstructing its shell at the small, pinued and, anc asit grows, it continuously lengtheas and widenssthc shell, by adding further coils or whars. Most smal shells are right-handed spirals, although a few twist naturally to the left, lfyou hold a snail shell with the opening downward. and look at it from above, you will find that roarly all of your al clockwise the up l the opening. This is what is meant by a right- harded spiral. A snail's shell is usually large enough or Lhe animal lo retreal within it completely. Most manne snails not only have this home to retreat into, but thcy alsohave a dor that they can use shut out the world. The door is a homy plug called the operaalum, which nearly seals offthe opening ofthe shell. The sna as we have seen, makes B lifolong job of oonstructing its portable home, When the snail dies, this home is noe wasted; however, for an emply snail shell can booomeadwelling pl ce ar a variety interesting creatures. The chief inhabitant of such shells is the hermit crab